As is generally well known in the art, the main metering system of a carburetor operates on the principle of a pressure differential between the carburetor fuel bowl and the induction passage at the venturi. That is, when the carburetor throttle is opened, the velocity of the air rushing past the induction passage venturi means is increased. This, in turn, decreases the pressure at the main metering nozzle located near the venturi, which results in a greater amount of rate of fuel being delivered into the intake or induction passage from the fuel bowl.
When the throttle is opened suddenly, however, the increase in the flow of fuel lags behind that of the flow of air due to the greater inertia of the heavier fuel. For this reason, carburetors are provided with an accelerating pump adapted to positively supply a predetermined extra amount of fuel when the throttle is opened; this helps to maintain a proper fuel-air ratio until the inertia of the fuel is overcome and the main metering system is back to normal operation.
In the past such accelerating pumps exhibited an objectionable characteristic in that they supplied the same amount of extra fuel at all engine temperatures. The volatility of the fuel varies considerably with change in temperature. Thus, in the past, an accelerating pump which was adjusted to provide just the right amount of extra fuel when the engine (and/or fuel) was cold, provided too much fuel when the engine was relatively hot. Likewise, if such an accelerating pump was set to provide the correct amount of additional fuel when the engine was relatively hot, there was a deficiency of fuel when the engine was cold.
Heretofore, the prior art attempted to overcome the problem of being able to provide differing quantities of additional acceleration fuel for respective relatively cold and hot engine conditions. To that end the prior art, heretofore, had proposed the employment of abutment means with such abutment means being intended to change the effective pump stroke depending on the temperature of the fuel. The abutment means, at least in one form suggested by the prior art, comprised a movable thermostatic member (moved to either of two operating conditions dependent on the sensed temperature of the fuel), carried as by the displaceable portion of the pump structure, which coacted with a cooperating fixed stop.
Although such prior art structures did, to some degree, overcome the problem of providing only a fixed additional amount of fuel regardless of engine (and/or fuel) temperature, such structures have been found difficult to manufacture and also difficult to calibrate. In this respect, the amount of additional fuel during cold engine operation as well as the amount of additional fuel during hot engine operation was totally determined by the same thermostatic member; accordingly, it sometimes became difficult, if not impossible, to provide desired rates of additional fuel flow where such rates were beyond one of the limits intrinsically determined or established by the thermostatic member.
Accordingly, the invention as herein disclosed and claimed is primarily directed to the solution of the aforementioned as well as related and attendant problems.